1. Field of the Invention
The present invention pertains to a type of projection exposure apparatus for exposing a mask pattern on a photosensitive substrate in a photolithography process. Such a process is used for manufacturing semiconductor elements, liquid-crystal display elements, pickup elements (CCDs), or film-type magnetic heads.
2. Description of Related Art
In a photolithographic process for manufacturing semiconductor elements, liquid-crystal display elements, pickup elements (CCDs, etc.), or film-type magnetic heads, a projection exposure apparatus, such as a "stepper", is used for transferring the image of a pattern formed on the reticle as a mask onto a wafer or glass plate coated with a photoresist. The transfer operation is performed by a projection optical system. In order to enlarge the pattern without increasing a load on the projection optical system, a step-and-scan method, in which an exposure is taken with synchronized scanning of the reticle and wafer with respect to the projection optical system, has been adopted.
In a projection exposure apparatus, the illuminating light emitted from a light source, such as an excimer laser with a wavelength of 248 nm or 193 nm, typically passes through a shaping lens, an illuminating field stop (reticle blind), a condenser lens, mirrors, and so on. The light is irradiated on a reticle held on a reticle stage. The imaging light beam passes through the reticle, goes through a projection optical system, which has been optimally corrected for aberrations, and exposes the wafer to an image of the reticle pattern.
In such a projection exposure apparatus, before the exposure operation, it is necessary to align the reticle and the wafer very precisely. In order to perform this alignment, an alignment mark is formed on the wafer, as a position detection mark, during a preceding operation step. By detecting the position of this alignment mark, it is possible to detect the correct position of the wafer or, more particularly, the circuit pattern on the wafer.
There are three primary alignment method types. The first of these types is one in which an alignment microscope or other alignment sensor, entirely separated from the projection optical system, is utilized. Such an alignment sensor is used to perform position detection of the alignment mark and is referred to as an "off-axis" type of sensor. Usually, in order to prevent exposure of the photoresist on the wafer to be exposed, the wavelength of the light beam for alignment is kept in a region to which the photoresist is insensitive. Such a wavelength is typically 550 nm or longer. In an off-axis type sensor, it is possible to make use of an optical system optimized to the alignment wavelength with respect to the alignment sensor. Consequently, it is possible to realize a detection system which is free from the chromatic aberration problems which will be mentioned below. However, the alignment sensor is entirely separate from the projection optical system. Due to certain factors, such as thermal expansion caused by changes in the temperature of the body of the projection exposure, the relative positions of the projection optical system and the alignment sensor will vary. This leads to alignment errors.
The other two alignment method types make use of the projection optical system itself as a portion of the alignment sensor optical system. The other methods utilize a TTL (Through-the-Lens) type alignment sensor and a TTR (Through-the-Reticle) type alignment sensor, respectively.
In the TTL type alignment sensor, alignment is performed by using the projection optical system as the optical path of the light beam at the alignment wavelength. Detection is performed without the aid of the reticle, however, and the light beam is guided outside the imaging optical path by curved mirrors and so on.
When using the TTR type alignment sensor, an alignment mark on the reticle and an alignment mark on the wafer are optically superimposed (imaged). Detection is performed directly. Consequently, the optical path of the light beam at the alignment wavelength goes through the reticle.
In an alignment operation using the TTL and TTR alignment sensor types, with the projection optical system itself used as a portion of the optical system of the alignment sensor, detection is carried out by using the projection optical system after optimizing with respect to the imaging light beam (ultraviolet light). As a result, chromatic aberration becomes a problem for the projection optical system at the alignment wavelength.
Efforts have been made to correct for chromatic aberration. Such efforts include the addition of a chromatic correcting part for the light beam at the alignment wavelength in the projection optical system and performing correction for chromatic aberration on the light beam as it is guided outside of the imaging light beam of the pattern by using a bent mirror. Since the projection optical system itself is used as a part of the optical system of the alignment sensor, adverse influences due to thermal expansion and so on become very small. It is possible to realize alignment with high precision and high stability.
When alignment is performed using an off-axis type sensor or a TTL type alignment sensor, it is necessary to measure a baseline. The baseline indicates a relationship between the projected position of the image, at the exposure wavelength of the pattern formed on the reticle and the position of the detection center of the alignment sensor. The error in this measurement becomes an error in alignment. In addition to alignment using the off-axis type sensor, for alignment using the TTL type alignment sensor, variation in the measurement data may take place after baseline checking.
With regard to alignment using the TTR type alignment sensor, since the alignment mark on the wafer is directly detected with respect to the alignment mark on the reticle, the alignment is immune to influences from various sources of error which accompany the alignment. This type of sensor can be regarded as having the highest precision.
Alignment can be performed by using the TTR type alignment sensor to correct for the aberration of the projection optical system while an exposure wavelength is in the UV region. However, in this case, for an alignment light beam with wavelength of 550 nm or longer, a chromatic aberration correction element is mandatory. This is inconvenient.
Conventional chromatic aberration correction not only affects the aberration correction of the alignment light beam, but also exacerbates the aberration of the imaging light beam. In addition, since there is a tendency to use shorter wavelengths for exposure as the pattern of the semiconductor IC becomes finer, the difference in wavelength between the imaging light beam and the alignment light beam increases even further. It becomes even more difficult, therefore, to perform the chromatic aberration correction.